1. An Initiative to Tame the Plasma Material Interface R.J. Goldston, J.E. Menard, J.P. Allain, J.N. Brooks, J.M. Canik R. Doerner, G.-Y. Fu, D.A. Gates, C.A. Gentile, J.H. Harris, A. Hassanein, N.N. Gorelenkov, R. Kaita, S.M. Kaye, M. Kotschenreuther, G.J. Kramer, H.W. Kugel, R. Maingi, S.M. Mahajan, R. Majeski, C.L. Neumeyer, R.E. Nygren, M. Ono, L.W. Owen, S. Ramakrishnan, T.D. Rognlien, D.N. Ruzic, D.D. Ryutov, S.A. Sabbagh, C. H. Skinner, V.A. Soukhanovskii, T.N. Stevenson, M.A. Ulrickson, P.M. Valanju, R.D. Woolley Columbia, LLNL, ORNL, PPPL, Purdue, SNL, UCSD, U. Ill, U. Texas

2. Demo Presents a Much Larger PMI Challenge than ITER Higher heat flux, longer pulses, higher duty factor 4x ITER’s heat flux 5000x longer pulses 5x higher even short-term average duty factor Erosion, dust production, tritium retention and component lifetime issues are much more challenging due to Demo’s mission. Demo must show practical solutions that allow for continuous operation for at least 2 full-power years between PFC change outs. ITER plans to change out divertors after ~ 0.08 full-power years – at much lower power. Many solutions used on ITER are not Demo-relevant Moderate fraction of radiated power Water cooled ~200C plasma facing components Intermittent dust collection and tritium clean-up

3. DRAFT Questions for an Initiative to Tame the Plasma-Material Interface Can high-performance, fully steady-state plasma operation, with Demo- relevant plasma-wall interactions, avoid high-energy ELMs and damaging disruptions? Can extremely high radiated-power fraction be consistent with high confinement and acceptable (n D +n T )/n e ? Can magnetic flux expansion and/or stellarator-like edge ergodization reduce heat loads sufficiently, consistent with adequate He pumping? Can tungsten or other solid materials provide acceptable erosion rates, core radiation and tritium retention? Can dust production be limited, and can dust be removed? Can liquid surfaces effectively handle high heat flux and provide adequate tritium exhaust, while limiting dust production? Can plasma-material interface solutions developed at low neutron fluence be made compatible with the high neutron fluence of Demo?

4. DRAFT Required Initiative Activities Materials and Technology Development Develop new refractory PFC materials and test in both powerful PMI machine and under neutron irradiation Develop and test PFC technologies for solid systems, including PFM to heat sink joining and He cooling with O and T removal Develop and test liquid PFC technologies, including modeling and experimental validation, and techniques for recycling evaporated lithium Develop technologies for real-time dust removal Existing Confinement Experiments Develop predictive understanding of power scrape-off Develop non-inductive scenarios without ELMs and disruptions Test innovative divertor configurations and PFC materials (both solid and liquid) Develop extensive diagnostics for plasma material interaction Theory and Computation Increase theory and computation focus on edge and SOL physics Advance theory of plasma-material interaction, including surface properties under erosion and redeposition Design new plasma-facing alloys and model liquid metals Design coil systems for stellarator-like edge / MHD stability Develop new refractory PFC materials and test in both powerful PMI machine and under neutron irradiation.  A New Hot-Wall, High-Power Confinement Facility

5. The First-wall Heat-flux Challenge ~ P/S The Divertor Heat-flux Challenge ~ P/R Power scrape-off width at low gas fueling, mapped from divertor plate to outer midplane, does not vary systematically with machine size. Loarte: 1999 Fundamenski: 2004 Maingi: 2007, 2008 New Goldston Scaling: ~ 8mm?

6. P h /R 50 MW/m and P in /S 1 MW/m 2 are Required ~ > ~ < P in = P aux + P  P h = P in - P brem - P sync For high Q systems, P h ~ (2/3) P in W/R ~ 5 MJ/m allows experiments to control ELMs and disruptions, without unacceptable PFC damage.

7. EU-B: Z eff = 2.7 n/n g = 1.2 f rad = 80 - 90% H H = 1.2 R 0 = 8.6m I p = 28MA P = 1.33 Gw e EU studies High P h / P LH is Needed to Test Highly Radiative Solution Can fusion plasmas operate with very high radiated power to reduce divertor heat flux, while maintaining good performance? Physics test requires input power exceeding H-mode threshold power by a large factor since much of the radiated power comes from within the separatrix Planned long-pulse experiments do not quite match EU-B, ARIES-RS or an ITER-like Demo P h /P LH @ n = n G –KSTAR (29 MW)4.1 –EAST (24 MW)5.2 –JT-60SA (41 MW)3.1 –ITER (120 MW) 2.2 –EU-B (653 MW)5.8 –ARIES RS (340 MW)5.6 –ITER-like Demo (400 MW)7.3 (Y.R. Martin FEC 2004, eq. 7)

8. Poloidal Field Flexibility is Needed to Test Flux Expansion SN vs. DN allows strong variation in heat flux Flux expansion has a dramatic effect on peak heat flux How does this extrapolate to very high power levels? NSTX

9. Tungsten Alloys must be Tested, but it is not Certain they will Work At high fluence and wall temperature, dust and foam are serious concerns Require capability to monitor and remove dust during operation Testing must be at Demo conditions, including wall temperature He-induced foam Dust source Nagoya University UCSD

10. The Ability to Test Liquid Metal Divertor Solutions is Needed FTU, Italy Capillary Porous System (CPS) T max ≤ 600C > 10 MW/m 2 in T-11 Successful tests of lithium in TFTR, T-11, FTU, CDX-U, NSTX NSTX to test liquid lithium divertor Reduces recycling, improves confinement E-beam test to 25 MW/m 2 for 5 - 10 minutes, 50 MW/m 2 for 15s. Plasma focus test to 60 MJ/m 2 off-normal load Direct route to tritium removal, no dust, no damage?

11. Long Pulses and Hot Walls are Needed to Study Tritium Retention Pulse length should be 200 – 1000 sec, to minimize transient effects Tritium retention time in Demo (inventory/flow) < 10 4 sec Tore Supra DITS experiment ~ 2 x 10 4 sec Total on-time ~ 10 6 sec / year provides for up to ten 10 5 sec tritium retention studies, ~10x H diffusion time in 5mm W @ 1000K Trace tritium capability is highly desirable Experiments must be done at Demo wall temperature ~ 1000K. Tore Supra Carbon PFCs Phase 2 Phase 1 Puff - Pump Temperature (K). Diffusivity (m 2 /sec) of H in W

12. Unprecedented Access for Diagnostics, Heating, Current Drive and PFC Services is Critical Extensive view in toroidal and poloidal angle of all plasma-material interactions Extensive in-situ surface analysis capabilities Extensive PFC engineering performance measurements A full set of advanced confinement, stability and sustainment diagnostics for high-performance operation A full set of advanced heating, current drive and control systems for high- performance operation without damaging ELMs or disruptions PFC services for heating, cooling and pumping will require substantial access Tore Supra, France ICRF antenna

13. DRAFT Technical Requirements for a New Hot-Wall, High-Power Confinement Facility Input power / plasma surface area <~ 1 MW/m2 Input power / major radius >~ 50 MW/m Heating power / H-mode threshold power > 6, at n = n G Stored energy / major radius ~ 5 MJ/m Flexible poloidal field system capable of wide variation in flux expansion and ability to divert field lines to large R Non-axisymmetric coils to produce stellarator-like edge and improve MHD stability High temperature ~ 1000K first wall operational capability Replaceable first wall and divertor Pulse length ~ 200 – 1000 sec; total on-time ~ 10^6 sec / year Unprecedented access for surface and plasma diagnostics, PFC services Deuterium and trace tritium operational capability Synergy with a Fusion Materials Irradiation Facility